Personal display with vision tracking

ABSTRACT

A display apparatus includes an image source, an eye position detector, and a combiner, that are aligned to a user&#39;s eye. The eye position detector monitors light reflected from the user&#39;s eye to identify the pupil position. If light from the image source becomes misaligned with respect to the pupil, a physical positioning mechanism adjusts the relative positions of the image source and the beam combiner so that light from the image source is translated relative to the pupil, thereby realigning the display to the pupil. In one embodiment, the positioner is a piezoelectric positioner and in other embodiments, the positioner is a servomechanism or a shape memory alloy.

TECHNICAL FIELD

[0001] The present invention relates to displays and, more particularly,to displays that produce images responsive to a viewer's eyeorientation.

BACKGROUND OF THE INVENTION

[0002] A variety of techniques are available for providing visualdisplays of graphical or video images to a user. For example, cathoderay tube type displays (CRTs), such as televisions and computer monitorsare very common. Such devices suffer from several limitations. Forexample, CRTs are bulky and consume substantial amounts of power, makingthem undesirable for portable or head-mounted applications.

[0003] Flat panel displays, such as liquid crystal displays and fieldemission displays, may be less bulky and consume less power. However,typical flat panel displays utilize screens that are several inchesacross. Such screens have limited use in head mounted applications or inapplications where the display is intended to occupy only a smallportion of a user's field of view.

[0004] More recently, very small displays have been developed forpartial or augmented view applications. In such applications, a portionof the display is positioned in the user's field of view and presents animage that occupies a region 42 of the user's field of view 44, as shownin FIG. 1. The user can thus see both a displayed image 46 andbackground information 48.

[0005] One difficulty with such displays is that, as the user's eyemoves to view various regions of the background information, the user'sfield of view shifts. As the field of view shifts, the position of theregion 42 changes relative to the field of view 44. This shifting may bedesirable where the region 42 is intended to be fixed relative to thebackground information 48. However, this shifting can be undesirable inapplications where the image is intended to be at a fixed location inthe user's field of view. Even if the image is intended to move withinthe field of view, the optics of the displaying apparatus may notprovide an adequate image at all locations or orientations of the user'spupil relative to the optics.

[0006] One example of a small display is a scanned display such as thatdescribed in U.S. Pat. No. 5,467,104 of Furness et. al., entitledVIRTUAL RETINAL DISPLAY, which is incorporated herein by reference. Inscanned displays, a scanner, such as a scanning mirror or acousto-opticscanner, scans a modulated light beam onto a viewer's retina. Thescanned light enters the eye through the viewer's pupil and is imagedonto the retina by the cornea and eye lens. As will now be describedwith reference to FIG. 2, such displays may have difficulty when theviewer's eye moves.

[0007] As shown in FIG. 2, a scanned display 50 is positioned forviewing by a viewer's eye 52. The display 50 includes four principalportions, each of which will be described in greater detail below.First, control electronics 54 provide electrical signals that controloperation of the display 50 in response to an image signal V_(IM) froman image source 56, such as a computer, television receiver,videocassette player, or similar device.

[0008] The second portion of the display 50 is a light source 57 thatoutputs a modulated light beam 53 having a modulation corresponding toinformation in the image signal V_(IM). The light source may be adirectly modulated light emitter such as a light emitting diode (LED) ormay be include a continuous light emitter indirectly modulated by anexternal modulator, such as an acousto-optic modulator.

[0009] The third portion of the display 50 is a scanning assembly 58that scans the modulated beam 53 of the light source 57 through atwo-dimensional scanning pattern, such as a raster pattern. One exampleof such a scanning assembly is a mechanically resonant scanner, such asthat described U.S. Pat. No. 5,557,444 to Melville et al., entitledMINIATURE OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, which isincorporated herein by reference. However, other scanning assemblies,such as acousto-optic scanners may be used in such displays.

[0010] Optics 60 form the fourth portion of the display 50. The imagingoptics 60 in the embodiment of FIG. 2 include a pair of lenses 62 and 64that shape and focus the scanned beam 53 appropriately for viewing bythe eye 52. The scanned beam 53 enters the eye 52 through a pupil 65 andstrikes the retina 59. When scanned modulated light strikes the retina59, the viewer perceives the image.

[0011] As shown in FIG. 3, the display 50 may have difficulty when theviewer looks off-axis. When the viewer's eye 52 rotates, the viewer'spupil 65 moves from its central position. In the rotated position all ora portion of the scanned beam 53 from the imaging optics 56 may notenter the pupil 65. Consequently, the viewer's retina 59 does notreceive all of the scanned light. The viewer thus does not perceive theentire image.

[0012] One approach to this problem described employs an optics thatexpand the cross-sectional area of the scanned effective beam. A portionof the expanded beam strikes the pupil 65 and is visible to the viewer.While such an approach can improve the effective viewing angle and helpto ensure that the viewer perceives the scanned image, the intensity oflight received by the viewer is reduced as the square of the beamradius.

SUMMARY OF THE INVENTION

[0013] A display apparatus tracks the orientation or position of auser's eye and actively adjusts the position or orientation of an imagesource or manipulates an intermediate component to insure that lightenters the user's pupil or to control the perceived location of avirtual image in the user's field of view. In one embodiment, thedisplay includes a beam combiner that receives light from a backgroundand light from the image source. The combined light from the combiner isreceived through the user's pupil and strikes the retina. The userperceives an image that is a combination of the virtual image and thebackground.

[0014] In addition to the light from the background and light from theimage source, additional light strikes the user's eye. The additionallight may be a portion of the light provided by the image source or maybe provided by a separate light source. The additional light ispreferably aligned with light from the beam combiner. Where theadditional light comes from a source other than the image source, theadditional light is preferably at a wavelength that is not visible.

[0015] A portion of the additional light is reflected or scattered bythe user's eye and the reflected or scattered portion depends in partupon whether the additional light enters the eye through the pupil orwhether the additional light strikes the remaining area of the eye. Thereflected or scattered light is then indicative of alignment of theadditional light to the user's pupil.

[0016] In one embodiment, an image field of a detector is aligned withthe light exiting the beam combiner. The detector receives the reflectedportion of the additional light and provides an electrical signalindicative of the amount of reflected light to a position controller.

[0017] In one embodiment, the detector is a low-resolution CCD array andthe position controller includes an electronic controller and a look uptable in a memory that provides adjustment data in response to thesignals from the detector. Data from the look up table drives apiezoelectric positioning mechanism that is physically coupled to asubstrate carrying both the detector and the image source.

[0018] When the detector indicates a shift in location of the reflectedadditional light, the controller accesses the look up table to retrievepositioning data. In response to the retrieved data, the piezoelectricpositioning mechanism shifts the substrate to realign the image sourceand the detector to the pupil.

[0019] In another embodiment, the CCD array is replaced by aquadrant-type detector, including a plurality of spaced-apart detectors.The outputs of the detectors drive a control circuit that implements asearch function to align the scanned beam to the pupil.

[0020] In one embodiment, imaging optics having a magnification greaterthan one helps to direct light from the image source and additionallight to the user's eye. Physical movement of the image source anddetector causes an even greater movement of the location at which lightfrom the image source strikes the eye. Thus, small movements induced bythe piezoelectric positioning mechanism can track larger movements ofthe pupil position.

BRIEF DESCRIPTION OF THE FIGURES

[0021]FIG. 1 is a diagrammatic representation of a combined imageperceived by a user resulting from the combination of light from animage source and light from a background.

[0022]FIG. 2 is a diagrammatic representation of a scanner and a user'seye showing alignment of a scanned beam with the user's pupil.

[0023]FIG. 3 is a diagrammatic representation of a scanner and a user'seye showing misalignment of the scanned beam with the user's pupil.

[0024]FIG. 4 is a diagrammatic representation of a display according toone embodiment of the invention including a positioning beam anddetector.

[0025]FIG. 5 is an isometric view of a head-mounted scanner including atether.

[0026]FIG. 6 is a diagrammatic representation of the display of FIG. 4showing displacement of the eye relative to the beam position andcorresponding reflection of the positioning beam.

[0027]FIG. 7A is a diagrammatic representation of reflected lightstriking the detector in the position of FIG. 4.

[0028]FIG. 7B is a diagrammatic representation of reflected lightstriking the detector in the position of FIG. 6.

[0029]FIG. 8 is a diagrammatic representation of the display of FIG. 2showing the image source and positioning beam source adjusted to correctthe misalignment of FIG. 6.

[0030]FIG. 9 is a detail view of a portion of a display showing shapememory alloy-based positioners coupled to the substrate.

[0031]FIG. 10 is a schematic of a scanning system suitable for use asthe image source in the display of FIG. 4.

[0032]FIG. 11 is a top plan view of a position detector including fourseparate optical detectors.

[0033] FIGS. 12A-C are diagrammatic representations of a displayutilizing a single reflective optic and a moving optical source.

[0034]FIG. 13 is a top plan view of a bi-axial MEMS scanner for use inthe display of FIG. 2.

[0035]FIG. 14 is a diagram of an alternative embodiment of a displayincluding an exit pupil expander and a moving light emitter.

[0036]FIG. 15A is a diagrammatic representative of nine exit pupilscentered over an eye pupil.

[0037]FIG. 15B is a diagrammatic representation of shifting of the eyepupil of FIG. 15A and corresponding shifting of the exit pupil array.

DETAILED DESCRIPTION OF THE INVENTION

[0038] As shown in FIG. 4, a virtual retinal display 70 according to theinvention includes control electronics 72, a light source 74, a scanningassembly 58, and imaging optics 78. As with the embodiment of FIG. 2,the light source may be directly or indirectly modulated and the imagingoptics 78 are formed from curved, partially transmissive mirrors 62, 64that combine light received from a background 80 with light from thescanning assembly 58 to produce a combined input to the viewer's eye 52.The light source 74 emits light modulated according to image signalsV_(IM) the image signal source 56, such as a television receiver,computer, CD-ROM player, videocassette player, or any similar device.The light source 74 may utilize coherent light emitters, such as laserdiodes or microlasers, or may use noncoherent sources such as lightemitting diodes. Also, the light source 74 may be directly modulated oran external modulator, such as an acousto-optic modulator, may be used.One skilled in the art will recognize that a variety of other imagesources, such as LCD panels and field emission displays, may also beused. However, such image sources are usually not preferred because theytypically are larger and bulkier than the image source described in thepreferred embodiment. Their large mass makes them more difficult toreposition quickly as described below with reference to FIGS. 6-8.Moreover, although the background 80 is presented herein as a“real-world” background, the background light may be occluded or may beproduced by another light source of the same or different type.

[0039] Although the elements here are presented diagrammatically, oneskilled in the art will recognize that the components are typicallysized and configured for mounting to a helmet or similar frame as ahead-mounted display 67, as shown in FIG. 5. In this embodiment, a firstportion 71 of the display 67 is mounted to a head-borne frame 73 and asecond portion 75 is carried separately, for example in a hip belt. Theportions 71, 75 are linked by a fiber optic and electronic tether 77that carries optical and electronic signals from the second portion tothe first portion. An example of a fiber coupled scanner display isfound in U.S. Pat. No. 5,596,339 of Furness et. al., entitled VIRTUALRETINAL DISPLAY WITH FIBER OPTIC POINT SOURCE which is incorporatedherein by reference. One skilled in the art will recognize that, in manyapplications, the light source may be coupled directly to the scanningassembly 58 so that the fiber can be eliminated.

[0040] Returning to the display 70 of FIG. 4, the user's eye 52 istypically in a substantially fixed location relative to the imagingoptics 78 because the display 70 is typically head mounted. For clarity,this description therefore does not discuss head movement in describingoperation of the display 70. One skilled in the art will recognize thatthe display 70 may be used in other than head-mounted applications, suchas where the display 70 forms a fixed viewing apparatus having an eyecupagainst which the user's eye socket is pressed. Also, the user's headmay be free for relative movement in some applications. In suchapplications, a known head tracking system may track the user's headposition for coarse positioning.

[0041] Imaging optics 78 redirect and magnify scanned light from thescanning assembly 58 toward the user's eye 52, where the light passesthrough the pupil 65 and strikes the retina 59 to produce a virtualimage. At the same time, light from the background 80 passes through themirrors 62, 64 and pupil 65 to the user's retina 59 to produce a “real”image. Because the user's retina 59 receives light from both the scannedbeam and the background 80, the user perceives a combined image with thevirtual image appearing transparent, as shown in FIG. 1. To ease theuser's acquisition of light from partially or fully reflective mirrors62, 64, the imaging optics 78 may also include an exit pupil expanderthat increases the effective numerical aperture of the beam of scannedlight. The exit pupil expander is omitted from the Figures for clarityof presentation of the beam 53.

[0042] In addition to light from the light source 74, the imaging optics78 also receive a locator beam 90 from an infrared light source 92carried by a common substrate 85 with the light source 74. Though thelocator beam 90 is shown as following a different optical path forclarity of presentation, the infrared light source 92 is actuallypositioned adjacent to the light source 74 so that light from the lightsource 74 and light from the infrared light source 92 are substantiallycollinear. Thus, the output of the imaging optics 78 includes light fromthe infrared light source 92. One skilled in the art will recognizethat, although the infrared light source 92 and the light source 74 areshown as being physically adjacent, other implementations are easilyrealizable. For example, the infrared light source 92 may be physicallyseparated from the light source 74 by superimposing the locator beam 90onto the light from the light source 74 with a beam splitter andsteering optics.

[0043] Tracking of the eye position will now be described with referenceto FIGS. 6-9. As shown in FIG. 6, when the user's eye 52 moves, thepupil 65 may become misaligned with light from the light source 74 andinfrared light source 92. All or a portion of the light from the lightsource 74 and infrared source 92 may no longer enter the pupil 65 or mayenter the pupil 65 at an orientation where the pupil 65 does not directthe light to the center of the retina 59. Instead, some of the lightfrom the sources 74, 92 strikes a non-pupil portion 96 of the eye. As isknown, the non-pupil portion 96 of the eye has a reflectance differentand typically higher than that of the pupil 65. Consequently, thenonpupil portion 96 reflects light from the sources 74, 92 back towardthe imaging optics 78. The imaging optics 78 redirect the reflectedlight toward an optical detector 98 positioned on the substrate 85adjacent to the sources 74, 92. In this embodiment, the detector 98 is acommercially available CCD array that is sensitive to infrared light. Aswill be described below, in some applications, other types of detectorsmay be desirable.

[0044] As shown in FIG. 7A, when the user's eye is positioned so thatlight from the sources 74, 92 enters the pupil (i.e., when the eye ispositioned as shown in FIG. 4), a central region 100 of the detector 98receives a low level of light from the imaging optics 78. The area oflow light resulting from the user's pupil will be referred to herein asthe pupil shadow 106. When the eye 52 shifts to the position shown inFIG. 6, the pupil shadow shifts relative to the detector 88 as shown inFIG. 7B.

[0045] The detector data, which are indicative of the position of thepupil shadow 106 are input to an electronic controller 108, such as amicroprocessor or application specific integrated circuit (ASIC).Responsive to the data, the controller 108 accesses a look up table in amemory device 110 to retrieve positioning data indicating an appropriatepositioning correction for the light source 74. The positioning data maybe determined empirically or may be calculated based upon known geometryof the eye 52 and the scanning assembly 58.

[0046] In response to the retrieved positioning data, the controller 110activates X and Y drivers 112, 114 to provide voltages to respectivepiezoelectric positioners 116, 118 coupled to the substrate 85. As isknown, piezoelectric materials deform in the presence of electricalfields, thereby converting voltages to physical movement. Therefore, theapplied voltages from the respective drivers 112, 114 cause thepiezoelectric positioners 116, 118 to move the sources 74, 92, asindicated by the arrow 120 and arrowhead 122 in FIG. 8.

[0047] As shown in FIG. 8, shifting the positions of the sources 74, 92shifts the locations at which light from the sources 74, 92 strikes theuser's eye, so that the light once again enters the pupil. The pupilshadow 106 once again returns to the position shown in FIG. 7A. Oneskilled in the art will recognize that the deformation of thepiezoelectric positioner 116 is exaggerated in FIG. 8 for demonstrativepurposes. However, because the mirrors 62, 64 have a magnificationgreater than one, small shifts in the position of the substrate 85 canproduce larger shifts in the location at which the light from the lightsource 74 arrives at the eye. Thus, the piezoelectric positioners 112,114 can produce sufficient beam translation for many positions of theeye. Where even larger beam translations are desirable, a variety ofother types of positioners, such as electronic servomechanisms may beused in place of the piezoelectric positioners 112, 114. Alternatively,shape memory alloy-based positioners 113, such as equiatomicnickel-titanium alloys, can be used to reposition the substrate as shownin FIG. 9. The positioners 113 may be spirally located, as shown in FIG.9 or may be in any other appropriate configuration. One skilled in theart will also recognize that the imaging optics 78 does not alwaysrequire magnification, particularly where the positioners 116, 118 areformed from a mechanism that provides relatively large translation ofthe scanner 70.

[0048]FIG. 10 shows one embodiment of a mechanically resonant scanner200 suitable for use as the scanning assembly 58. The resonant scanner200 includes as the principal horizontal scanning element, a horizontalscanner 201 that includes a moving mirror 202 mounted to a spring plate204. The dimensions of the mirror 202 and spring plate 204 and thematerial properties of the spring plate 204 are selected so that themirror 202 and spring plate 204 have a natural oscillatory frequency onthe order of 1-100 kHz. A ferromagnetic material mounted with the mirror202 is driven by a pair of electromagnetic coils 206, 208 to providemotive force to mirror 202, thereby initiating and sustainingoscillation. Drive electronics 218 provide electrical signal to activatethe coils 206, 208.

[0049] Vertical scanning is provided by a vertical scanner 220structured very similarly to the horizontal scanner 201. Like thehorizontal scanner 201, the vertical scanner 220 includes a mirror 222driven by a pair of coils 224, 226 in response to electrical signalsfrom the drive electronics 218. However, because the rate of oscillationis much lower for vertical scanning, the vertical scanner 220 istypically not resonant. The mirror 222 receives light from thehorizontal scanner 201 and produces vertical deflection at about 30-100Hz. Advantageously, the lower frequency allows the mirror 222 to besignificantly larger than the mirror 202, thereby reducing constraintson the positioning of the vertical scanner 220.

[0050] In operation, the light source 74, driven by the image source 56(FIG. 8) outputs a beam of light that is modulated according to theimage signal. At the same time, the drive electronics 218 activate thecoils 206, 208, 224, 226 to oscillate the mirrors 202, 222. Themodulated beam of light strikes the oscillating horizontal mirror 202,and is deflected horizontally by an angle corresponding to theinstantaneous angle of the mirror 202. The deflected light then strikesthe vertical mirror 222 and is deflected at a vertical anglecorresponding to the instantaneous angle of the vertical mirror 222. Themodulation of the optical beam is synchronized with the horizontal andvertical scans so that at each position of the mirrors, the beam colorand intensity correspond to a desired virtual image. The beam therefore“draws” the virtual image directly upon the user's retina. One skilledin the art will recognize that several components of the scanner 200have been omitted for clarity of presentation. For example, the verticaland horizontal scanners 201, 220 are typically mounted in fixed relativepositions to a frame. Additionally, the scanner 200 typically includesone or more turning mirrors that direct the beam such that the beamstrikes each of the mirrors a plurality of times to increase the angularrange of scanning.

[0051]FIG. 11 shows one realization of the position detector 88 in whichthe CCD array is replaced with four detectors 88A-88D each aligned to arespective quadrant of the virtual image. When the user's eye 52 becomesmisaligned with the virtual image, the pupil shadow 106 shifts, asrepresented by the broken lines in FIG. 10. In this position, theintensity of light received by one or more of the detectors 88A-88Dfalls. The voltage on the positioners 116, 118 can then be varied torealign the scanned light to the user's eye 52. Advantageously, in thisembodiment, the outputs of the four quadrant detector can form errorsignals that, when amplified appropriately, may drive the respectivepositioners 114, 116 to reposition the light emitter 74.

[0052] A further aspect of the embodiment of the display 70 of FIG. 8 isz-axis adjustment provided by a third positioner 128 that controls theposition of the light source 74 and scanner 76 along a third axis. Thethird positioner 128, like the X and Y positioners 114, 116 is apiezoelectric positioner controlled by the electronic controller 108through a corresponding driver 130.

[0053] As can be seen from FIG. 8, when the user's eye 52 rotates toview an object off-axis and the X and Y positioners 116, 118 adjust theposition of the light source 74, the distance between the scanner 76 andthe first mirror 64 changes slightly, as does the distance between thefirst mirror 64 and the eye 52. Consequently, the image plane defined bythe scanned beam may shift away from the desired location and theperceived image may become distorted. Such shifting may also produce aneffective astigmatism in biocular or binocular systems due to differencein the variations between the left and right eye subsystems. Tocompensate for the shift in relative positions, the controller 108,responsive to positioning data from the memory 110, activates the thirdpositioner 130, thereby adjusting the z-axis position of the lightsource 74. The appropriate positioning data can be determinedempirically or may be developed analytically through optical modeling.

[0054] One skilled in the art will also recognize that the controller108 can also adjust focus of the scanned beam 53 through the thirdpositioner 130. Adjustment of the focus allows the controller tocompensate for shifts in the relative positions of the scanning assembly76, mirrors 62, 64 and eye 52 which may result from movement of the eye,temperature changes, pressure changes, or other effects. Also, thecontroller 108 can adjust the z-axis position to adapt a head-mounteddisplay to different users.

[0055] Although the embodiments herein are described as havingpositioning along three orthogonal axes, the invention is not solimited. First, physical positioning may be applied to other degrees ofmotion. For example, rotational positioners may rotate the mirrors 62,64, the light source 74 or the substitute 85 about various axes toprovide rotational positioning control. Such an embodiment allows thecontroller log to establish position of the virtual image (e.g. theregion 42 of FIG. 1). By controlling the position of the virtual image,the controller 108 can move the region 42 to track changes in the user'sfield of view. The region 42 can thus remain in a substantially fixedposition in the user's field of view. In addition to rotational freedom,one skilled in the art will recognize that the three axes are notlimited to orthogonal axes.

[0056] While the embodiments described herein have included two mirrors62, 64, one skilled in the art will recognize that more complex or lesscomplex optical structures may be desirable for some applications. Forexample, as shown in FIGS. 12A-C, a single reflective optics 300 can beused to reflect light toward the viewer's eye 52. By tracing the opticalpaths 302 from the scanning assembly 58 to the pupil 65, thecorresponding position and angular orientation of the scanning assembly58 can be determined for each eye position, as shown in FIGS. 12A-C.

[0057] The determined position and orientation are then stored digitallyand retrieved in response to detected eye position. The scanningassembly 58 is then moved to the retrieved eye position and orientation.For example, as shown in FIG. 12B, when the field of view of the eyes iscentered, the scanning assembly 58 is centered. When the field of viewis shifted left, as shown in FIG. 12A, the scanning assembly 58 isshifted right to compensate.

[0058] To reduce the size and weight to be moved in response to thedetected eye position, it is desirable to reduce the size and weight ofthe scanning assembly 58. One approach to reducing the size and weightis to replace the mechanical resonant scanners 200, 220 with amicroelectromechanical (MEMS) scanner, such as that described in U.S.Pat. No. 5,629,790 entitled MICROMACHINED TORSIONAL SCANNER toNeukermans et. al. and U.S. Pat. No. 5,648,618 entitled MICROMACHINEDHINGE HAVING AN INTEGRAL TORSION SENSOR to Neukermans et. al., each ofwhich is incorporated herein by reference. As described therein andshown in FIG. 13, a bi-axial scanner 400 is formed in a siliconsubstrate 402. The bi-axial scanner 400 includes a mirror 404 supportedby opposed flexures 406 that link the mirror 404 to a pivotable support408. The flexures 406 are dimensioned to twist torsionally therebyallowing the mirror 404 to pivot about an axis defined by the flexures406, relative to the support 408. In one embodiment, pivoting of themirror 404 defines horizontal scans of the scanner 400.

[0059] A second pair of opposed flexures 412 couple the support 408 tothe substrate 402. The flexures 412 are dimensioned to flex torsionally,thereby allowing the support 408 to pivot relative to the substrate 402.Preferably, the mass and dimensions of the mirror 404, support 408 andflexures 406, 412 are selected such that the mirror 404 resonates, at10-40 kHz horizontally with a high Q and such that the support 408pivots at frequencies that are preferably higher than 60 Hz, although insome applications, a lower frequency may be desirable. For example,where a plurality of beams are used, vertical frequencies of 10 Hz orlower may be acceptable.

[0060] In a preferred embodiment, the mirror 404 is pivoted by applyingan electric field between a plate 414 on the mirror 404 and a conductoron a base (not shown). This approach is termed capacitive drive, becauseof the plate 414 acts as one plate of a capacitor and the conductor inthe base acts as a second plate. As the voltage between platesincreases, the electric field exerts a force on the mirror 404 causingthe mirror 404 to pivot about the flexures 406. By periodically varyingthe voltage applied to the plates, the mirror 404 can be made to scanperiodically. Preferably, the voltage is varied at the mechanicallyresonant frequency of the mirror 404 so that the mirror 404 willoscillate with little power consumption.

[0061] The support 408 may be pivoted magnetically or capacitivelydepending upon the requirements of a particular application. Preferably,the support 408 and flexures 412 are dimensioned so that the support 408can respond frequencies well above a desired refresh rate, such as 60Hz.

[0062] An alternative embodiment according to the invention, shown inFIG. 14 includes a diffractive exit pupil expander 450 positionedbetween the scanning assembly 58 and the eye 52. As described in U.S.Pat. No. 5,701,132 entitled VIRTUAL RETINAL DISPLAY WITH EXPANDED EXITPUPIL to Kollin et. al. which is incorporated herein by reference, ateach scan position 452, 454 the exit pupil expander 450 redirects thescanned beam to a plurality of common locations, to define a pluralityof exit pupils 456. For example, as shown in FIG. 15A, the exit pupilexpander 450 may produce nine separate exit pupils 456. When the user'spupil 65 receives one or more of the defined exit pupils 456, the usercan view the desired image.

[0063] If the user's eye moves, as shown in FIG. 15B, the pupil 65 stillmay receive light from one or more of the exit pupils 456. The user thuscontinues to perceive the image, even when the pupil 65 shifts relativeto the exit pupils 456. Nevertheless, the scanning assembly 58 (FIGS.12A-12C) shifts, as indicated by the arrows 458 in FIG. 14 and arrows460 in FIG. 15B to center the array of exit pupils 456 with the user'spupil 65. By re-centering the array relative to the pupil 65, the numberof exit pupils 456 can be reduced while preserving coupling to the pupil65.

[0064] Although the invention has been described herein by way ofexemplary embodiments, variations in the structures and methodsdescribed herein may be made without departing from the spirit and scopeof the invention. For example, the positioning of the various componentsmay also be varied. In one example of repositioning, the detector 88 andinfrared source 92 may be mounted separately from the light source 74.In such an embodiment, the detector 98 and infrared source 92 may bemounted in a fixed location or may be driven by a separate set ofpositioners. Also, in some applications, it may be desirable toeliminate the infrared source 92. In such an embodiment, the detector 98would monitor reflected visible light originating from the light source74. Also, the infrared beam and scanned light beam may be made collinearthrough the use of conventional beam splitting techniques. In stillanother embodiment, the piezoelectric positioners 116, 118 may becoupled to the mirror 64 or to an intermediate lens 121 to produce a“virtual” movement of the light source 74. In this embodiment,translation of the mirror 64 or lens 121 will produce a shift in theapparent position of the light source 74 relative to the eye. Byshifting the position or effective focal length of the lens 121, thelens 121 also allows the display to vary the apparent distance from thescanner 200, 400 to the eye 52. For example, the lens 121 may be formedfrom or include an electro-optic material, such as quartz. The effectivefocal length can then be varied by varying the voltage across theelectro-optic material for each position of the scanner 200, 400.Moreover, although the horizontal scanners 200, 400 are described hereinas preferably being mechanically resonant at the scanning frequency, insome applications the scanner 200 may be non-resonant. For example,where the scanner 200 is used for “stroke” or “calligraphic” scanning, anon-resonant scanner would be preferred. One skilled in the art willrecognize that, although a single light source is described herein, theprinciples and structures described herein are applicable to displayshaving a plurality of light sources. In fact, the exit pupil expander450 of FIG. 14 effectively approximates the use of several lightsources. Further, although the exemplary embodiment herein utilizes thepupil shadow to track gaze, a variety of other approaches may be withinthe scope of the invention, for example, reflective techniques, suchknown “glint” techniques as may be adapted for use with the describedembodiments according to the invention may image the fundus or featuresof the iris to track gaze. Accordingly, the invention is not limitedexcept as by the appended claims.

What is claimed is:
 1. A method of producing an image for viewing by aneye, comprising the steps of: emitting light from a first location;modulating the light in a pattern corresponding to the image; producinga positioning beam; directing the positioning beam along a first pathtoward the eye; receiving a portion of light reflected from the eye withan optical detector; producing an electrical signal responsive to thereceived reflected light; identifying a pupil position responsive to theelectrical signal; and physically repositioning the first location inresponse to the electrical signal.
 2. The method of claim 1 wherein animage source produces the light and wherein the step of physicallyrepositioning the first location in response to the electrical signalincludes physically repositioning the image source relative to theuser's eye.
 3. The method of claim 2 wherein the step of physicallyrepositioning the image source includes activating a piezoelectricpositioner coupled to the image source.
 4. The method of claim 3 whereinthe step of physically repositioning the image shown includes activatinga shape memory alloy coupled to the image source.
 5. The method of claim1 wherein the optical detector includes a detector array and wherein thestep of producing an electrical signal responsive to the receivedreflected light includes outputting data from the detector array.
 6. Themethod of claim 1 wherein the positioning beam is an infrared beam. 7.The method of claim 1 wherein the step of producing an electrical signalincludes the steps of: outputting data from the detector array;retrieving data stored in a memory; and producing the electrical signalin response to the retrieved data.
 8. The method of claim 1 wherein aportion of the emitted light forms the positioning beam.
 9. The methodof claim 1 wherein the step of emitting light includes producing thelight with an image source and guiding the light with guiding optics andwherein the step of physically repositioning the first location inresponse to the electrical signal includes physically varying therelative positioning of the guiding optics and the image source.
 10. Themethod of claim 9 wherein the guiding optics include a lens.
 11. Themethod of claim 10 wherein the guiding optics further include a turningreflector.
 12. A method of producing an image in response to an imagesignal for perception by a user, comprising the steps of: emitting, froma first position, light corresponding to the image responsive to theimage signal; directing the emitted light corresponding to the imagetoward the user's eye; determining an eye position while directing theemitted light corresponding to the image toward the user's eye; andresponsive to the determined eye position adjusting the first positionto direct the emitted light toward the user's pupil.
 13. The method ofclaim 12 wherein the step of determining the eye position includes thesteps of: emitting a tracking beam of light; directing the tracking beamof light toward the user's eye; and monitoring light reflected from theuser's eye.
 14. The method of claim 13 wherein the step of emitting atracking beam of light includes the steps of emitting the tracking beamfrom substantially the first position.
 15. The method of claim 12wherein the step of monitoring light reflected from the user's eyeincludes: positioning an optical detector adjacent to the firstposition; and receiving a portion of the reflected light with thedetector.
 16. The method of claim 12 wherein the step of directing theemitted light corresponding to the image toward the user's eye includesscanning the emitted light with a scanner.
 17. The method of claim 16wherein the step of directing the tracking beam of light toward theuser's eye includes scanning the tracking beam with the scanner.
 18. Amethod in a display apparatus of identifying alignment of an opticalsource with an eye, comprising the steps of: projecting light from atracking source onto the eye; receiving light reflected from a pluralityof locations on the eye; generating electrical signals corresponding tothe received reflected light; responsive to the electrical signals,identifying a region of the eye having a reduced reflectance relative toother regions of the eye; and comparing the identified region of reducedreflectance with a reference region corresponding to centering of theoptical source relative to the reduced reflectance region.
 19. Themethod of claim 18 further including the step of aligning the trackingsource in a substantially fixed position relative to the optical source.20. The method of claim 18 wherein the step of receiving light reflectedfrom a plurality of locations on the eye includes receiving lightreflected from a plurality of locations on the eye with a photodetector.21. The method of claim 20 wherein the photodetector is atwo-dimensional detector array.
 22. The method of claim 21 wherein thetwo-dimensional detector array is a CCD array.
 23. The method of claim20 wherein the photodetector includes a plurality of integrateddetectors.
 24. A method of aligning a virtual image to an eye,comprising the steps of: directing image light from a first locationalong a first set of optical paths to the eye produce the virtual image;directing a tracking beam of light toward the eye such that a portion ofthe tracking beam is reflected from the eye; receiving a reflectedportion of the tracking beam with a photodetector; producing anelectrical signal in response to the reception of the reflected portion;responsive to the electrical signal, identifying a region of thereflected portion corresponding to a pupil; determining an adjustment offirst location that increases the amount of image light entering thepupil; and adjusting the first location responsive to the determinedadjustment.
 25. The method of claim 24 wherein the display includes animage source that produces the image light and a detector that producesthe electrical signal, and wherein the image source and detector aremounted to a common supporting body.
 26. The method of claim 25 whereinthe step of adjusting the first set of optical paths responsive to thedetermined adjustment includes moving the supporting body.
 27. Themethod of claim 26 wherein the step of moving the supporting bodyincludes activating a piezoelectric positioner.
 28. The method of claim27 wherein the step of moving the supporting body includes activating ashape memory alloy.
 29. A virtual display for producing an image forviewing by a user's eye, comprising: an image source operative to emitlight in a pattern corresponding to the image along a path toward theuser's eye; an optical detector aligned to the user's eye and operativeto detect a location of a region of the user's eye having a reflectancecorresponding to a selected eye feature having a predetermined positionrelative to a pupil of the eye, the optical detector producing a signalindicative of the detected location; and a positioning mechanism havinga control input coupled to the optical detector and a positioning outputcoupled to the image source, the positioning mechanism being responsiveto the signal indicative of the detected location to physicallyreposition the image source in a direction that shifts the optical pathto the pupil.
 30. The display of claim 29 wherein the positioned is anelectrically actuated positioner and wherein the signal indicative ofthe detected location is an electrical signal.
 31. The display of claim29 wherein the image source includes a light emitter and imaging opticsconfigured for relative repositioning by the positioning mechanism. 32.The display of claim 29 wherein the image source and detector aremounted to a common supporting body.
 33. The display of claim 29 whereinthe positioning mechanism is coupled to the common body to physicallydisplace the common body.
 34. The display of claim 29 wherein the imagesource is a retinal scanner.
 35. The display of claim 29 furthercomprising a beam combiner having a first input aligned to the imagesource and a second input, the beam combiner being operative to directlight from the first and second inputs and to provide the combined lightto a user's retina.
 36. A display apparatus including eye positiontracking, comprising: a first scanner; beam-turning optics aligned tothe eye; an image source mounted to a base and aligned to beam-turningoptics at an angle selected to direct light from the image source to theeye; an optical source aligned to the eye; a detector aligned to the eyeand responsive to output an electrical signal indicative of alignment ofthe optical source relative to a selected region of the eye; and apositioning mechanism coupled to the base and responsive to theelectrical signal from the detector to physically adjust the relativepositions of the base relative and the beam-turning optics.
 37. Thedisplay apparatus of claim 36 wherein the image source is a retinalscanner.
 38. The display apparatus of claim 36 wherein the positioningmechanism is a piezoelectric positioner.
 39. The display apparatus ofclaim 36 wherein the positioning mechanism is a servomechanism.
 40. Thedisplay apparatus of claim 36 wherein the positioning mechanism includesa shape memory alloy.
 41. The display apparatus of claim 36 whereinbeam-turning optics includes a beam combiner.
 42. The display apparatusof claim 41 wherein the beam combiner includes an optical magnifier. 43.The display apparatus of claim 42 wherein the optical magnifier is amirror.
 44. The display apparatus of claim 40 wherein the beam combinerincludes a beam splitter.
 45. The display apparatus of claim 36 furtherincluding a head mounting structure carrying the optical source, thebeam-turning optics, and the positioning mechanism.
 46. A displayapparatus, comprising a light movable source operative to emit a beam oflight modulated according to a derived image, the movable light sourcebeing responsive to a position input to vary the effective position ofthe beam of light, an exit pupil expander positioned to receive theemitted beam of light, the exit pupil expander being responsive to emita plurality of exit beams in response to the received beam of light; aneye tracker oriented to detect a user's eye position and configured tooutput an electric signal corresponding to the detected eye position; apositioner having an electrical input coupled to the eye tracker toreceive the electric signal, the positioner further being coupled to thelight source, the positioner being operative to provide the positioninput in response to the electrical signal.
 47. The display apparatus ofclaim 46 wherein the exit pupil expander is a diffractive element.